Power plants that use renewable energy can produce a large amount of energy in optimal conditions but the produced amount of energy decreases when the working conditions are not optimal. For example, such power plants can be wind farms or solar plants.
For this reason, these power plants are usually provided with energy storage systems.
Traditional energy storage systems can for example include a pump station for pumping water in a basin; the water can then be conveyed into a water turbine when energy is needed.
Another energy storage system includes a compressor, for compressing a gas (usually air) in a cave. This compressed air is then expanded in an air turbine when energy is needed.
FIGS. 1 and 2 show an example of such an energy storage system.
FIG. 1 shows the components used during compression of the gas and storage thereof in the cave and FIG. 2 shows the components used when energy is needed and the gas stored in the cave is expanded to gather mechanical power.
FIG. 1 shows a compressor 1, thermal energy storage 2 and the cave 3. During operation gas (for example air from the atmosphere) is compressed by the compressor 1 and is then cooled to be then stored in the cave 3.
The thermal energy storage 2 includes a heat exchanger 4, a cold tank 5 and a hot tank 6. The cold tank 5 and hot tank 6 contain a thermal storage medium that is used to store heat. In fact, during compression of the gas, the thermal storage medium passes from the cold tank 5 through the heat exchanger 4 to the hot tank 6, where it is accumulated. While passing through the heat exchanger 4 the thermal storage medium cools the gas and increases its own temperature.
FIG. 2 shows the cave 3 connected to the thermal energy storage 2 that in turn is connected to a turbine 7. During operation the compressed gas contained in the cave 3 is conveyed through the heat exchanger 4 to the turbine 7. When the gas passes through the heat exchanger 4, the thermal storage medium passes from the hot tank 6 through the heat exchanger 4 to the cold tank 5, to increase the temperature of the gas and reduce its own temperature. Within the turbine 7 the high pressure and temperature gas is expanded (to gather mechanical power for example to activate an electric generator) and the exhaust gas is discharged for example into the atmosphere.
The thermal storage medium must be able to conveniently operate in a very broad range of temperatures. For example, in traditional applications the temperature of the cold tank 5 is typically the atmospheric temperature, and the temperature of the hot tank 6 can be as high as about 550° C. or more. In order to have high efficiency, the thermal storage medium must exchange heat over the whole temperature range.
Nevertheless, finding a thermal storage medium that can be conveniently used over the whole temperature range is not easy, because the temperature range is very broad.